1. Field of the Invention
The present invention relates to a method for manufacturing heat-resistant and flame-retardant polyvinyl chloride fiber by grafting acrylic acid or its divalent metal salts onto polyvinyl chloride by irradiating the fiber with an ionizing radiation in the presence of ethylene dichloride, water and acrylic acid or in the presence of ethylene dichloride, methanol, water and calcium acrylate.
2. Description of the Prior Art
Polyvinyl chloride fiber has many advantages as a synthetic fiber, among which the flame retardance is an almost unique one. However, it has also a drawback, that the heat resistance is so low that the fiber shows remarkable heat shrinkage even below 100.degree. C. The most practical method to raise the heat-resistance i.e. the temperature of heat shrinkage is to use random copolymer of vinyl chloride and acrylonitrile in a ratio about 60:40. Such a fiber is commercially produced. The inventors of the present invention have previously presented an invention providing a method for improving the heat-resistance of polyvinyl chloride fiber by graft-copolymerizing acrylonitrile onto polyvinyl chloride. The graft-copolymer fiber shows even at 300.degree. C. no shrinkage. However, it was necessary to graft-copolymerize acrylonitrile in an amount about 100% by weight based on the starting polyvinyl chloride fiber in order to render the fiber sufficiently heat-resisting. Therefore, the flame-resistance of the polyvinyl chloride fiber was to a certain extent lost owing to such a large amount of the graft-copolymerized acrylonitrile.
On the other hand, it is known by those skilled in the art that the heat-shrinking of the polyvinyl chloride fiber can be mitigated by radiation grafting of acrylic acid, especially by further treatment of the graft fiber with a metallic salt. (See, for example, Tsuji, Ikeda and Kurokawa: Sen-i Gakkai-shi, vol. 23, page 335, 1967). However, mention is found neither on the flame-retardance of the graft fiber nor on the direct grafting of calcium acrylate onto polyvinyl chloride, and the method of rapid and effective grafting was not critically studied.